The present invention relates to seals between generator stator bars and their end fitting connections, and to a method for sealing a stator bar end fitting to prevent or reduce corrosion due to coolant water flowing through the fitting and the stator bar.
Water-cooled stator bars for electrical generators are comprised of a plurality of small rectangular solid and hollow copper strands brazed to one another to form a bar. The ends of the strands are brazed to an end fitting, typically referred to as stator bar clip. The end fitting serves as both an electrical and a cooling flow connection for the stator bar.
The hollow end fitting typically includes an enclosed chamber for ingress or egress of stator bar cooling liquid, typically deionized water. At one open end, the end fitting receives the ends of the strands of the stator bar. The fitting and the peripherally outermost copper strands of the stator bar are brazed to one another. The opposite end of the fitting is connected to a stator cooling conduit, e.g., a house.
Over time, leaks can developed about the connection between the stator bar ends and the stator bar fitting as well as between adjacent strands. It is believed that the leak mechanism is due to a two-part corrosion process which initiates in the braze alloy at the interior surface of the braze joint. Stagnant water in the chamber of the fitting has in the past been in contact with the braze alloy and the copper strands. This coolant contact with the braze joint and cooper strands is believed to cause corrosion and consequent leakage.
Field repair of coolant leaks through the stator bar end connections has been moderately successful. A leak site is identified by external visual examination of where the strands enter the end fitting. External visual leak detection indirectly identifies the leak site because the visible external leak on the outside of the bar and/or fitting may be a considerable distance away from a point within the fitting where the leak begins. This is particularly true if the leak is occurring between the strands of the stator bar.
Once a leak is identified by visible inspection, a vacuum is drawn inside the fitting and stator bar. An anaerobic cement is applied externally or reflow the braze material around the suspected leak sites. The vacuum suction draws the cement inwardly towards the leak path. However, it has been found that this cement repair method is not 100% effective in repairing the leak and is therefore considered only a temporary repair.
As an alternative to applying cement to seal a leak, an epoxy barrier coating has been applied to provide protection against water initiated corrosion mechanisms along the brazed length of the strand package. The epoxy barrier coating technique is disclosed in U.S. Pat. No. 5,605,590. The epoxy barrier coating has been an improvement over earlier corrosion protection techniques. However, the epoxy barrier coating may deteriorate when exposed to extreme temperatures and can be difficult to inspect in certain stator clip designs.
There is a need for an improved barrier coating for stator bar clips. In particular there is a need for corrosion protection between the stator bars and their clips. The corrosion protection should be robust and be applicable to various stator bar clip designs, including clips for raised hollow strand stator designs.